1. Field of the Invention
The present invention relates to an improvement in a digital exchange having a plurality of digital trunks connected to digital lines and to an improvement in a control method thereof and, more particularly, to a digital exchange which can suppress the failure of the exchange even when an abnormality or abnormalities occur in any of digital trunks from which clocks are extracted through the associated digital lines so long as at least one of the digital trunks is normal and also to a control method thereof.
2. Description of the Prior Art
An exchange generally functions to exchange interconnection between a plurality of extension telephone sets, connection of the extention telephone sets to central office lines (outside lines), connection of the central office lines to the extension telephone sets, and so on. There has been recently suggested a digital exchange which comprises digital trunks (which will be referred to as the T1 trunk, hereinafter) and functions to exchange digital data. FIG. 1 shows an arrangement of this type of digital exchange (which will be referred to merely as the exchange, hereinafter). In the drawing, an exchange 105 has T1 trunks 106, 107, 108, . . . which are connected at their one ends to a time switch 109 and also connected at the other ends to a central office 101 through a channel unit 102 acting as an interface with the central office 101 and through a T1 network 115. The time switch 109 is also connected with an extension telephone set 111 through an extension interface 110. In the illustrated example, 1.544 Mbps digital lines 112, 113, 114, . . . are connected between the T1 trunks 106, 107, 108, . . . and the channel unit 102, these trunks employing an interface called a DA format therebetween. In practical applications, a plurality of such channel units 102 are provided to be connected to the common central office 101 through the T1 network 115 being synchronized with respect to frequency.
The DS1 format employed in the digital line 112, 113, 114, . . . are shown in FIGS. 2(a) and (b), which is constituted of 12 frames as a multi-frame configuration each corresponding to 125 .mu.s and having 24 channels multiplexed. Each frame has a synchronization bit provided in its heading part and each channel consists of 8 bits least significant one LSB of which is allocated as a signaling bit. It is the T1 trunks 106, 107, 108, . . . that establish interface with the digital lines 112, 113, 114, . . .
Referring to FIG. 3, there is shown a detailed arrangement of the exchanged of FIG. 1 having the plurality of T1 trunks, in which a data bus 207 for transmitting control data to the T1 trunks 106, 107, 108, . . . therethrough as well as a PCM bus 206 for transmitting voice data subjected to a PCM (pulse code modulation) to the T1 trunks therethrough are provided to be connected to the T1 trunks 106, 107, 108, . . . The PCM bus 206 is also connected to the time switch 109. The time switch 109 performs the exchange of the PCM voice data between the trunks 106, 107, 108, . . . and connection therebetween. More specifically, the T1 trunk 106 extracts a DS1 clock signal 205 from the digital line 112 and sends it to the time switch 109 which in turn is operated in synchronism with the received DS1 clock signal. In the illustrated example, the DS1 clock signal 205 as an output of the T1 trunk 106 must be synchronized with a synchronization clock of the PCM bus 206 with respect to frequency. To this end, it is necessary for the time switch 109 to be synchronized with the DS1 clock signal 205. In this way, the DS1 clock signal 205 extracted at the T1 trunk 106 is used for the above frequency synchronization.
The details of the T1 trunk 106 are shown in FIG. 4, in which a clock extracting circuit 310 extracts the DS1 clock signal 205 on the basis of a signal received through a receiver 308 from the digital line 112 and sends the extracted clock signal 205 to the time switch 109. The data signal of the digital line 112 received through the receiver 308 is also applied to a rate conversion part 307 to be converted thereat to a 2 Mbps signal which is then supplied to a signaling-signal extracting part 306 and an alarm detector 311.
Meanwhile, with respect to the data signal on the PCM bus 206, channels (ch1, ch2, . . . , and ch32) are multiplexed as synchronized with a 2.048 MHz PCM clock signal at a rate of 2.048 Mbps in the pulse-to-pulse period of one frame corresponding to 125 .mu.s. The manner is shown in FIGS. 5(a) to (e). More in detail, FIG. 5(a) shows the 125 .mu.s frame pulse, FIG. 5(b) shows the 2.048 MHz PCM clock signal, FIG. 5(c) shows the PCM bus data, FIG. 5(d) shows the channels of the PCM bus, and FIG. 5(e) shows an 8 KHz synchronzation clock signal. Accordingly, it is necessary to absorb or arrange a difference in transmission rate between the digital line 112 and the PCM bus 206. For the purpose of arranging the rate difference, FIFOs 301 and 305 are connected to the PCM bus 206.
A 2 Mbps signal converted at the rate converter 307 is subjected at the signaling signal extracter 306 to an extraction to obtain a signaling signal that is then sent to the PCM bus 206 through the FIFO 305.
A PCM voice signal to be transmitted from the PCM bus 206 to the central office, on the other hand, is supplied through the FIFO 301 to a signaling-signal inserting part 302 to be subjected to a signaling insersion thereat and then applied to a rate coverter 303. The signal applied to the rate converter 303 is rate-converted thereat from the 2 Mbps signal to a 1.5 Mbps signal and then sent through a driver 304 onto a digital line 112.
In the event where the plurality of T1 trunks 106, 107, 108, . . . are connected to the channel unit 102 as shown in FIG. 1, data on the plurality of digital lines 112, 113, 114, . . . are mutually synchronized. Thus, the DS1 clock signal to be supplied to the time switch 109 in FIG. 3 is extracted at any one of the plurality of T1 trunks 106, 107, 108, . . . , and at the T1 trunk 106 in the example of FIG. 3 so that the extraction and output of the DS1 clock signal are not carried out at the other T1 trunks 107, 108, . . . And the time switch 109, which is shown in FIG. 6 in detail, receives the DS1 clock signal at a PLL circuit 401 which in turn sends it to a selector 404. The selector 404 also receives a clock signal from an emergency clock oscillator 403. The selector 404 usually selects the DS1 clock signal as a clock signal and sends it to a frequency divider 406. When the DS1 clock signal becomes abnormal, the selector 404 selects a clock signal from the oscillator 403 and sends it to the frequency divider 406. The clock signal received at the frequency divider 406 is frequency-divided to obtain a frame pulse 407 and a PCM clock 408 which are then applied to a speech memory 409 to establish synchronization between the transmission and reception of the PCM data.
In the event where some abnormality takes place in the digital line 112 and the T1 trunk 106 cannot extract the DS1 clock signal properly, the DS1 clock signal supplied to the time switch 109 becomes abnormal. At this time, the time switch 109 is operated with the clock signal from the emergency clock oscillator 403. However, this operation is for the purpose of saving various sorts of data necessary at the time of its restoration and thereafter the exchanging operation is stopped. For this reason, the above prior art digital exchange has had such a problem that, when some trouble occurs in the digital line 112 for the DS1 clock signal to be extracted or when the T1 trunk 106 acting to extract the DS1 clock signal becomes faulty, the whole operation of the exchange is stopped even if the other digital lines 113, 114, . . . and the other T1 trunks 107, 108, . . . are all normal.
The above T1 trunks are, in some cases, made respectively in the form of a digital trunk card to be arbitrarily plugged in and out. In this case, when one of the digital trunk cards playing a role of extracting the DS1 clock signal is erroneously plugged out for maintenance, it becomes impossible to extract the DS1 clock signal, thus involving the similar problem to in the above.